Watch: Clouds of money as airstrike hits ISIS bank

The ISIS terror group has cut its fighters' salaries by half, according to recently-leaked documents, in an indication of the immense pressure the jihadist groups is coming under from air assaults and losses on the ground.

The documents were circulated by the so-called Islamic State's treasury - known as Bayt Mal al-Muslimeen - in December, and justifies the drastic pay cut as a result of the "exceptional circumstances" the Islamist group finds itself under.

According to a translation provided by Middle East expert Aymenn Jawad Al-Tamimi, the document uses Koranic sources to claim that "jihad of wealth" (i.e. sacrificing one's money) is equal to "jihad of the soul" (i.e. martyrdom), and urges jihadists not to be "stingy."

"So on account of the exceptional circumstances the Islamic State is facing, it has been decided to reduce the salaries that are paid to all mujahideen by half, and it is not allowed for anyone to be exempted from this decision, whatever his position," the document reads.

It comes as the US-led air coalition continues to focus its strikes on ISIS economic and financial targets, placing considerable strain on the group's ability to finance its war effort.

The US Department of Defense released last week footage of one of the most significant single such strikes yet - the bombing of a major ISIS "bank" or "cash distribution center" in Mosul on January 11.

Footage of that strike was released several days later, and shows clouds of money fluttering to the ground and lining nearby rooftops (see video above).